The present application relates to a segmented transfer apparatus. In particular, the transfer apparatus should have at least two mutually adjacent segments. Each segment has a long stator linear motor and a control unit for controlling this linear motor.
In each of the segments, the associated control unit can ensure transportation of the workpiece carrier by applying a suitable current, typically an alternating current, to the linear motor. A difficulty arises in the transfer phase of the workpiece carrier from one segment to the other segment. This is because the two linear motors in the adjacent segments must then be synchronized. If the alternating current is sinusoidal, then an angle can be associated with each time, the so-called commutation angle. It is now important for the two linear motors to have currents applied to them for which the commutation angle is as accurately identical as possible. Furthermore, the total force caused by the currents should be constant during the transfer from one segment to the adjacent segment.
Until now, the synchronization has been carried out by a central control device instructing the control units of the individual segments to behave in a specific manner, by means of suitable control signals. Technically, the central control device defines the commutation angle and signals this to the two control units of the adjacent segments.
The use of a central control device does not make it possible to regulate the movement of the workpiece carrier. A nominal speed can be defined, but the actual speed then results on the basis of the control signals. It has been found that only a restricted speed range exists: the speed must be such that the static friction is overcome, while on the other hand, however, the workpiece carrier is prevented from tipping over. It is difficult to use control signals to accurately achieve a workpiece carrier movement at the optimum speed. The central control also means that a higher current level frequently occurs that is actually necessary, thus leading to the system heating.